Perchlorate Production by Photodecomposition of Aqueous Chlorine Solutions

摘要

Aqueous chlorine solutions (denned as chlorine solutions (Cl_(2,T)) containing solely or a combination of molecular chlorine (Cl_2), hypochlorous acid (HOG), and hypochlorite (OCl~-)) are known to produce toxic inorganic disinfection byproduct (e.g., chlorate and chlorite) through photoactivated transformations. Recent reports of perchlorate (ClO_4~-) production-a well-known thyroid hormone disruptor- from stored bleach solutions indicates the presence of unexplored transformation pathway(s). The evaluation of this potential ClO_4~- source is important given the widespread use of aqueous chlorine as a disinfectant. In this study, we perform detailed rate analysis of ClO_4~- generation from aqueous chlorine under varying environmental conditions including ultraviolet (UV) light sources, intensity, solution pH, and Cl_(2,T) concentrations. Our results show that ClO_4~- is produced upon UV exposure of aqueous chlorine solutions with yields ranging from 0.09 × 10~(-3) to 9.2 × 10~(-3)% for all experimental conditions. The amount of ClO_4~- produced depends on the starting concentrations of Cl_(2,T) and ClO_3~-, UV source wavelength, and solution pH, but it is independent of light intensity. We hypothesize a mechanistic pathway derived from known reactions of Cl_(2,T) photodecomposition that involves the reaction of Cl radicals with ClO_3~- to produce ClO_4~- with calculated rate coefficient (k_(ClO4-)) of (4-40) × 10s M~(-1) s~(-1) and (3-250) × 105 M~(-1) s~(-1) for UV-B/C and UV-A, respectively. The measured ClO_4~- concentrations for both UV-B and UV-C experiments agreed well with our model (R~2 = 0.88-0.99), except under UV-A light exposure (R~2 = 0.52-0.93), suggesting the possible involvement of additional pathways at higher wavelengths. Based on our results, phototransformation of aqueous chlorine solutions at concentrations relevant to drinking water treatment would result in ClO_4~- concentrations (～0.1 μg L~(-1)) much below the proposed drinking water limits. The importance of the hypothesized mechanism is discussed in relation to natural ClO_4~-formation by atmospheric transformations.